Method of improving the strength characteristics of paper prepared from partially acylated cellulose fibers



United States Patent 3,103,462 METHOD OF IMPROVING THE STRENGTH CHAR-ACTERISTICS OF PAPER PREPARED FROM PARTEALLY ACYLATED CELLULQSE FIBERSWilliam H. Griggs, Alan H. Gray, and Ralph D. Zatfrann,

Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., acorporation of New Jersey No Drawing. Filed Apr. 18, 1960, Ser. No.22,675 6 Claims. (Cl. 162-136) This invention relates to the manufactureof paper composed of partially esterified cellulose fibers improved instrength characteristics by sizing with styrene-maleic acid orstyrene-maleic anhydride type resins having therewith a cross-linkingagent and subsequently heat treating at a temperature within the rangeof 350400 P. such as with infrared heat or oven for a time short of thatwhich would cause discoloration of the paper.

Paper as ordinarily prepared is useful for, numerous purposes such asWrapping, writing paper, newsprint, printing paper, and many other ofthe common uses for that material. Paper has also been found to beuseful for special purposes but modifications in the usual paper makingprocesses to prepare paper for those specialy purposes are sometimesnecessary. For instance, it is desirable that paper for photographicpurposes exhibit good stability and resistance to imbibition of salts inprocessing operations. Papers subjected to aqueous baths have often beencharacterized by curling. This is particularly objectionable inphotographic processing and in the finished photographic products.

The application of a waterproof coating to paper for photographicpurposes such as the application of a cellulose ester coating theretohas been suggested. Oftentime waterproof paper exhibits a certain amountof stiffness and in many cases offers resistance to the application of acoating of an aqueous photographic emulsion thereon. The cost ofpreparing waterproofed paper for photographic purposes has often beenhigh which has precluded its use in commercial manufacture.

It has been discovered in recent years that paper comprising cellulosefibers chemically combined with lower fatty radicals exhibits propertiessuperior to those of paper prepared from conventional wood pulp fibers.For instance, paper prepared from cellulose fibers having a lower fattyacid radical content of l25%, apparent acetyl, exhibits good dimensionalstability and freedom from appreciable curl. Also, paper of this type ischaracterized by low water and salt retention in photographic processingoperations in comparison with many types of paper base which have beenemployed heretofore. It is desirable in the case of paper prepared frompartially acylated fibers that it in addition exhibit high strength andgood resistance to certain coating solvents. It is also desirable thatsuch a paper be resistant to size change when subjected to atmospherevarying in relative humidity or to the aqueous baths used inphotographic processing.

One object of our invention is to provide a method for treating paper ofpartially acylated fibers involving the use of heat whereby the strengthof that paper is increased. Another object of our invention is toprovide a method for treating paper of partially acylated fibers inwhich its resistance to the effects of certain coating solvents isenhanced. A further object of our invention is to provide a method fortreating paper in which the dimensional stability properties asrepresented by changes during wetting are enhanced. A still furtherobject of our invention is to prepare a partially acylated papercompatible with silver halide photographic emulsions. Other objects ofour invention will appear herein.

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The partially acylated paper treated in accordance with our invention iscomposed of -100% of partially acylated cellulose fiber, the cellulosefibers being chemically combined with fatty acid radicals of 2-4 carbonatoms which fibers then have a combined apparent acetyl content of15-25% In making the paper, the acyl-ated fiber is convenientlydispersed in water to form an aqueous slurry and beaten to a slownessuseful for the paper making operations. If any other fiber is used, itis present in an amount no more than 25% of the total fibers of thepaper. This other fiber may be a wood pulp or a rag fiber as commonlyused to make paper of sheet quality. After beating, the pulp slurry isapplied to the wire of a paper machine preferably after firstincorporating sizing therein such as described in Griggs and Zaifrannapplication Serial No. 22,711, filed on even date. After the paper isformed and desirably after drying, it may be tub sized with astyrene-maleic anhydride or styrenemaleic acid resin preferably afterthis resin has been mixed with a non-formaldehyde cross-linking agentsuch as a diepoxide resin. The paper is then subjected to a heattreatment such as that obtained by bringing the paper to a temperatureof 350400 F., preferably 350-375 F., for a period of at least 5 andpreferably 10 seconds but short of causing discoloration of the paper.

The method of sizing paper with the styrene resin materials is describedin application Serial No. 22,710 of ourselves, filed of even date.

In accordance with our invention, the paper is subjected to the 350-400F. temperature for a short time so as to avoid discoloration as mayoccur with prolonged heating. When the heating operation is carried outin an oven, the operation can be from 5 seconds up to 2 minutes orpossibly longer without discoloration occuring. When heating by infraredrays is used, the paper is heated up so much faster that the time ofheating should ordinarily be limited to less than a minute to avoiddiscoloration. If the heating is carried out by passing the paperbetween rolls heated to 350-400 F., the time of contact is ordinarilysufficiently short that discoloration is avoided.

We have found that by sizing partially acylated fiber paper and heattreating in accordance with our invention, substantial increases ofmullen and wet tensile strength are obtained and resistance to waterpenetration is somewhat better than with paper prepared from partiallyacylated cellulose fibers without the described sizing and heattreatment. in addition, there is a substantial improvement in thedimensional stability as indicated by the decrease in the wet expansion.

if paper is formed in the manner described above but sized with agelatin solution in preference to the styrenernaleic anhydride or acidresin solution, similar results may be realized but to a less extentthan with styrenemaleic anhydride resin sized paper.

The following examples illustrate the preparation of paper both with andwithout the utilization of our invention and a comparison of thecharacteristics obtained:

EXAMPLE 1 A roll of paper was prepared from cellulose fibers having acombined acetyl content of approximately 20% which had been internallysized with 7% starch, 2% of a cationic thermosettingpolyamide-epichl-orohydrin resin, as described in Example 1 of US.Patent No. 2,926,154, and /s of stearoyl hexadecylethenone (alkyl ketenedimer), all based on the weight of the fiber, dyes also having beenadded, in a manner similar to that described in Griggs and Zaffrannapplication Serial No. 786,041, filed January 12, 1959. Half of thisroll of paper was sized with 2% gelatin aqueous solution. The other halfof the roll of paper was also sized with a 2% gelatin solution but thenwas subjected to an infrared heat treatment in such a manner that thesheet temperature came to 350 F. in about seconds and remained at about350375 F. for another 5-10 seconds. The papers were both tested with theresults given in columns A and B of Table I which follows. It is to benoted that there was an increase of approximately 20% in wet tensilestrength, a substantial increase in penetration resistance and areduction in expansion, swell and bulk and in Cobb size of the infraredtreated paper (B) compared with the paper not subjected to 350 F.temperature.

EXAMPLE 2 A roll of paper was prepared from partially acetylated fibersin a manner similar to that described in the preceding example exceptthat the paper was sized with a mixture of 2% each of styrene-maleicanhydride and styrene-maleic acid resins as described in applicationSerial No. 22,710 of ourselves, filed of even date. This roll of paperwas water finished and one half was treated With infrared rays in such amanner that the sheet temperature came to about 350 F. in about 5seconds and remained at 350-375" F. for another 5-10 seconds. Columns Cand D of the table are the results obtained when testing the paper notsubjected to 350 F. temperature (C) and that subjected to infrared heat(D). It will be noted that in the case of the infrared heated paper ascompared with the paper not subjected to high temperature, there was a16% increase in wet tensile strength, a marked improvement indimensional stability as measured by reduction in expansion, swell andbulk and improved resistance to solvent eifeot as measured by decreasein Cobb size value.

EXAMPLE 3 A roll of paper was prepared from partially acetylated fibersin the manner described in Example 1. The paper was water finishedand'one half of the roll was heat treated in an oven at 350 F. for 2minutes. The results are given in columns A (not heat treated) and E(heat treated) in the following table. It will be noted that there was a25% increase in Mullen, a 33% increase in Wet tensile strength, asubstantial increase in penetration and a marked improvement indimensional stability as measured by a reduction in expansion, swell andbulk of the heated treated paper (E) over that which had been given noheat treatment (A).

EXAMPLE 4 A roll of paper was prepared from partially acetylated fibersin the manner described in Example 2. This roll of paper was waterfinished and one half was passed through an oven wherein it wassubjected to a treatment at 350 F. The results are given in column C(not heat treated) and column F (heat treated) in the following table.It will be noted that there is a 50% increase in wet tensile strengthand a marked improvement in dimensional stability as indicated by thereduction in expansion, swell and bulk of the heat treated paper (F)over that which had been given no heat treatment (C).

Table I Weight ghick. (dry Wet Te ile St 28 30 35 Penetration- 2, 000+2, 000+ 2,000+ Expansion 1. 30 1. 80 1. Fold:

Cobb Size 0.16

The swell and bulk values are obtained from the following formulas afterdetermining the dry thickness of the paper by soaking the paper for 20minutes in water at room temperature and in the case of the former,determining the wet thickness and in the case of the latter, thethickness upon redrying the paper to about 5% moisture content.

Wet thickdry thick Dry thick Wet, redried thickdry thick Dry thickEXAMPLE 5 Samples of partially acetylated paper which had beeninternally sized with aluminum stearate and cationic thermosettingpolyamide epichlorohydrin resin as described in Griggs and Zafirannapplication Serial No. 22,711, filed of even date, were surface sizedwith gelatin as described in Example 1 or with styrene-maleic anhydrideresin sizing as described in Example 2 and were subjected to an infraredheat treatment in such a manner that the sheet temperature came to about350 F. in about 5 seconds and remained at about 350-375 F. for another5-10 seconds. These papers exhibited characteristics superior to thoseof like paper which had been surface sized with gelatin or withstyrene-maleic anhydride resin but which had not been subjected to theheat treatment.

The paper prepared as described in accordance with our invention iseminently useful either with or without a baryta (BiaSO -I-gelatin)coating as a support for gelatino-silver halide photographic emulsions.In those cases Where the emulsion layers are unhardened, it is desirableto omit the gelatin size coating of the paper, the emulsion then beingapplied directly to the styrene-maleic resin size layer with or withouta preliminary baryta coating.

We claim:

1. A method of manufacturing paper of good strength characteristicswhich comprises preparing paper of fibers at least of which arecellulose fibers having an acyl content of 15-25%, apparent acetyl,surface sizing the thus prepared paper and subsequently subjecting thepaper to a temperature within the range of 350-400 F. for a time shortof that 'which will cause discoloration of the paper.

2. A method for the manufacture of paper having good strengthcharacteristics which comprises preparing a paper from cellulose fibersat least 75% of which are cellulose having an acyl content of 15-25%,apparent acetyl, surface sizing the paper with a styrene-maleic resinand subjecting the thus sized paper to a heat within the range of350-400" F. for a time short of that which will cause discoloration ofthe paper.

3. A method of manufacturing paper having good strength characteristicswhich comprises preparing paper from cellulose fibers at least 75% ofwhich are cellulose having an acyl content of 15-25%, apparent acetyl,surface sizing the paper with gelatin and subjecting the thus sizedpaper to a temperature within the range of 350- 400 F. for a time shortof that which will cause discolonation of the paper.

'4. A method of preparing paper having good strength characteristicswhich comprises preparing paper from cellulose fibers at least 75% ofwhich have been cellulose having an acyl content of 15-25%, apparentacetyl, surface sizing the paper with a sizing material selected fromthe group consisting :of gelatin and styrene-maleic resin andsubsequently subjecting the thus sized paper to treatment with infraredrays suificient to heat the paper to a temperature within the range of350-400 F. for a time short of that which would cause discoloration ofthe paper.

Percent swell= X Percent bulk: X 100 fibers at least 75% of which havebeen cellulose having an acyl content of 15-25%, apparent acetyl,surface sizing the thus formed paper with a sizing material selectedfrom the group consisting of gelatin and styrene-malcic resin andsubsequently treating the paper in an oven at a temperature within therange of 350-400" F. for a time short of that which will causediscoloration of the paper.

6. In the preparation of paper, in which paper, prepared cErom cellulosefibers having an acyl content of 1525%, apparent :acetyl, and internallysized with a cationic therrnosetting polyamide-epichlorohydrin resin, issurface sized with a sizing selected from the group consisting ofgelatin and styrene-m-aleic resin, the subsequent step which comprisessubjecting the paper to a tempenature within the range of 350-40 0 F.for a time short of that which causes discoloration of the paper.

References Cited in the file of this patent UNITED STATES PATENTS2,116,544 Schur May 10, 1938 2,577,624 Niles Dec. 4, 1951 2,887,429Griggs et a1 May 19, .1959 2,913,356 Schroeder Nov. 17, 1959 2,926,116Keirn Feb. 23, 1960 3,002,860 Bishop et a1. Oct. 3, 1961 FOREIGN PATENTS535,932 Great Britain Apr. 28, 1941

1. A METHOD OF MANUFACTURING PAPER OF GOOD STRENGTH CHARACTERISTICSWHICH COMPRISES PREPARING PAPER OF FIBERS AT LEAST 75% OF WHICH ARECELLULOSE FIBERS HAVING AN ACYL CONTENT OF 15-25%, APPARENT ACETYL,SURFACE SIZING THE THUS PREPARED PAPER AND SUBSEQUENTLY SUBJECTING THEPAPER TO A TEMPERATURE WITHIN THE RANGE OF 350-400*F. FOR A TIME SHORTOF THAT WHICH CAUSE DISCOLORATION OF THE PAPER.